All the scenes happen in the present in Beowulf, but with some slight variation. When he requests a meeting with the king and when he is challenged by Unferth, they both happen when the story is being told. Beowulf's foretelling of Grendel's defeat is happening in the present about something he will do in the future, obviously. But his story about Beca is something that he is telling, therefore it has already happened and it is told as a flashback.
C. Beowulf telling of his challenge with Breca